Super absorbent polymer (SAP) is synthetic polymer material that can absorb moisture of 500 to 1000 times of self-weight, and is differently named according to development companies such as super absorbency material (SAM), absorbent gel material (AGM), and the like. The superabsorbent polymer began to be commercialized as sanitary items, and currently, it is being widely used as hygienic goods such as a disposable diaper and the like, water-holding material for soil, water stop material for civil engineering and architecture, sheets for raising seedling, freshness preservatives in the field of food circulation, and formentation material, and the like.
As a method for preparing the superabsorbent polymer, a reverse phase suspension polymerization method or an aqueous polymerization method, and the like are known. The reverse phase suspension polymerization method is disclosed in, for example, Japanese Patent Laid-Open Publication No. Sho 56-161408, Japanese Patent Laid-Open Publication No. 57-158209, and Japanese Patent Laid-Open Publication No. 57-198714, and the like. As the aqueous polymerization method, a thermal polymerization method wherein hydrogel polymer is polymerized while breaking and cooling in a kneader equipped with several shafts, and a photopolymerization method wherein an aqueous solution of high concentration is simultaneously polymerized and dried by irradiating UV on a belt, and the like are known.
Meanwhile, the hydrogel polymer obtained by the polymerization reaction is generally dried and pulverized, and then, the surface of the polymer is selectively crosslinked so as to have desired absorption capacity and pressure absorption capacity. The selective crosslinking requires controlling of the penetration depth of a surface crosslinker, and for this, it is general that the penetration depth of a surface crosslinker is controlled using alcohol and water. However, if the penetration depth of a crosslinker is controlled using alcohol, there are disadvantages in that it is unfavorable in terms of the cost because a large amount of alcohol is consumed, and additional drying treatment is required.
Accordingly, to overcome these problems, methods of minimizing the use amount of alcohol or using no alcohol have been suggested in the prior art. However, there was a problem in that in case the use amount of alcohol is too lowered, water may be excessively absorbed in the polymer, thus making effective crosslinking difficult due to the agglomeration of gel.